A hydraulic rotary valve as used in a power steering apparatus comprises a valve rotor having a plurality of axial grooves formed therein, and a valve sleeve which is rotatably fitted around the valve rotor and having axial grooves, which are disposed in overlapping relationship with the opposite circumferential sides of a groove formed in the internal surface of the valve rotor. A relative rotational displacement between the valve rotor and the valve sleeve is effective to control the delivery to or discharge from a power cylinder of hydraulic fluid. Such a hydraulic rotary valve has a clearance which is commonly referred to as an "underlap" at its neutral position. Such underlap is designated as a clearance 7 in FIG. 4 where an overlapping relationship between an area 3 of a valve rotor 1 where no groove 2 is formed and an area 6 of a valve sleeve 4 where a groove 5 is not formed is avoided, or stated conversely, a fluid passage is defined by an overlapping portion of the groove 2 in the valve rotor 1 and the groove 5 in the valve sleeve 4. In the prior art practice, the magnitude of the underlap 7 has been determined by the width of the groove 2 formed in the valve rotor 1.
Specifically, in the prior art practice, when forming the groove 2 into the valve rotor 1, a milling machine is initially used to cut a groove 8 of a width which is slightly less than the final width of the groove 2, and then the opposite sidewalls 9 of the resulting groove are ground in order to improve the indexing accuracy and the precision of the groove width, thus utilizing two steps. Subsequently, the sidewall 9 of the groove 2 is chamfered as shown at 10 adjacent to the peripheral surface.
The underlap 7 in question is determined by the position of point 11 where the sidewall 9 of the groove 2 intersects with the chamfered portion 10. Because the grinding operation for the sidewall 9 of the groove represents a separate step from the chamfering operation, an error involved with the grinding operation, as indicated by an area shown enclosed by broken lines in FIG. 4, causes the position of the point 11 to change from time to time. As a consequence, when the magnitude of the underlaps 7 located on the opposite sides of the groove 2 are different, this causes a disadvantage that there results an undersirable hydraulic response which is unbalanced in the lateral direction.